The semiconductor industry had product sales of over $125 billion in 1997 and due to its size is a growth engine for the world economy. Nearly all of these products were manufactured from one single raw material-silicon. The industry has accumulated decades of experience and invested billions of dollars in developing manufacturing processes for silicon-based semiconductors.
Two drivers of the semiconductor industry are performance improvement and cost reduction. This has been achieved by shrinking the size of chips while at the same time putting more transistors on individual chips. Historically there has been doubling of the number of transistors on a chip every 18 month, this is known as Moore's law. Many predictions have been made that this rate of progress will necessarily decrease, but it has not. So far, engineers and scientists have found ways to go around roadblocks and continued the progress.
One unavoidable consequence of device shrinkage and higher operating speeds is an increased power density. However, in order to maintain the high operation speeds, or more precisely the high operation frequency, the operation temperatures of the chip must be kept at reasonable levels. Especially since an increased operation temperature also gives increased levels of noise and accordingly levels of erroneous operation. It has long been known that this problem can be solved by employing materials with a higher thermal conductivity and/or other beneficial properties than possessed by silicon. But since the semiconductor industry has invested such vast amount of experience and money into the existing production lines based on silicon-chips, any material innovations in the semiconductor industry should be built on, not replace, this vast silicon experience and manufacturing base. An example of this can be seen from the fact that gallium arsenide, a material with higher performances than silicon, has not been able to replace the silicon due to the necessity of investing billions of dollars to the change the manufacturing infrastructure world-wide.
Thus, in order to further increase the capacity, i.e. increase the density and the operating frequency of the next generation of integrated circuits, the heat produced in the silicon must be fast and efficiently extracted. Up till now this problem has been solved by “thinning”, i.e. the processors are made thinner and/or to equip them with heat sinks and on-board fans etc. in order to increase the heat removal rates. In practise, this solution can only be employed as long as the power consumption of the processor is less than 110-120 W, and this power level was expected to be reached around 2001. As far as we know, the only practically implemented solution to further increase the heat removal rate is introduction of closed-loop cooling systems in the processors. But they tend to cost more than the processor itself and are thus not a satisfactory solution.
It does therefore seem that the solution to the problem with excess heat should be found by changing the focus from how to lead the generated heat away, to how to avoid the heat from being generated.
The reason for heat generation is the electrical resistance within the silicon material of the processors. It has been known since early 1940 that the thermal conductivity of some elements was depended upon the isotope composition of the element. For instance, it became established in 1958 that the thermal conductivity of diamond was reduced by 30% by introduction of 1.1% of the 13C-isotope. Capinski et al. [1] and [2] has recently shown that isotopically pure 28Si has at least 60% better thermal conductivity at room temperature and over 250% better at −170° C., than silicon with a natural isotope composition. The reason for the enhanced thermal resistance in isotopically mixed metals is believed to be due to differences in the vibrational states of the different isotopes. Thus, if mono-isotopic silicon could be used, the vibration (phonon-) spectra throughout the crystalline structure would be greatly simplified and hence the thermal conductivity will be significantly improved [1 and 2]. A major microprocessor manufacturer has modelled isotopically pure 28Si-wafers based on the findings of Capinski et al., and calculated that the peak temperature of a 1 GHz microprocessor would be reduced by 35° C. Such a substantial reduction in the heat generation rate would remove a major industrial roadblock and allow the cheap and already implemented solution of equipping silicon chips with heat sinks and on-board fans to be used for years to come.
Another great advantage with employing isotopically pure silicon is that isotopically pure silicon is silicon. It is chemically almost identically the same as “natural silicon” and can readily be used in existing manufacturing processes without requiring changes in the production line of electronic devices. Another fortunate fact is that naturally occurring silicon contains three stable non-radioactive isotopes where 28Si is the far most abundant (˜92%), while the two others 29Si and 30Si are present in approximately 5 and 3%, respectively. Thus a large portion of the isotope separation is already performed by Mother Nature, such that the removal of the relatively small fractions of the two other isotopes should be relatively easy compared to other materials with a more equalised isotope composition.
Thus in conclusion, isotopically pure 28Si represents a very promising and easily implemented solution to the excess heat problem of high performance silicon chips.